I couldn't find ice cube trays with covers OR silicone trays but I did find some Wilson silicone mini muffin trays. they are exactly once ounce! this is great - they should pop right out of the mold.
It is weird, I was in Ikea this week with the same mission on my mind. Their silicone ice cube trays say "for water only". I wonder why? Where did you find yours?
Re: To prepare for solids...